A numerical and experimental study of a two-dimensional body with moonpool in waves and current

Doctoral thesis

Åpne

Permanent lenke

Utgivelsesdato

Metadata

Samlinger

Sammendrag

The present work has relevane for the behaviour of a vessel equipped with a moonpool in ocaean waves. A moonpool is an opening in the middle of the vessel used for various marine operations. Resonant wave oscillations in the moonpool with the water moving as a vertical piston occur and cause operational limits. It is again confirmed by both numerical calculations and experiments in idealized two-dimensional flow conditions that flow separation from the lower entrance to the moonpool has a clear limiting effect on the resonant moonpool oscillations. Furthermore, the influence of low forward velocity on the piston-mode amplitude has been investigated and found insignificant. By considering in coming waves on a freely-floating vessel, it is found that the water behavior in the moonpool has a significaant effect on the body motions in 2D-flow.

The present work is divided in two parts, first forced heave motion with and without low forward velocity of a two-dimensional body with moonpool has been considered. The second part is regular wave-induced behaviour of a floating stationary two-dimensional body with a moonpool, with focus on resonant piston-mode motion in the moonpool and rigid-body motions. Two separate dedicated two-dimensional experimental programmes have been performed. The outcome has been used to validate the two developed numerical hybrid methods that are used in the present work. The two hybrid methods are here named the "semi-nonlinear hybrid method" and the "nonlinear hybrid method", and both couple potential and viscous flow. The semi-nonlinear hybrid method uses linear free-surface and body-boundary conditions. The nonlinear hybrid method uses

fully nonlinear free-surface and exact body-boundary conditions.

The harmonic polynomial cell (HPC)method is used to solve the Laplace equation in the potential flow domain, while the finite volume method (FVM) is used to solve the Navier-Stokes equations in the viscous flow domain near the corners of the body.

Results from the two hybrid methods are compared with the experimental data. The nonlinear hybrid method compares well with the data, while certain discrepancies are observed for the semi-nonlinear method. In particular, the roll motion is

over-predicted by the semi-nonlinear hybrid method.

Error sources in the semi-nonlinear hybrid method are discussed and investigated in detail in a separate section.

The first part of the work involved both experimental and numerical study of selected parameters and their effect on the piston-mode amplitude during forced heave oscillations.

More precisely we investigated the effect of different drafts, moonpool edge profiles, heave oscillation amplitudes and low forward velocities.

In the second part regular wave-induced behaviour of a floating two-dimensional body with a moonpool was also studied by experimental and numerical methods.

Here we have investigated the effect of different wave steepnesses, vessel drafts and moonpool edge profiles on a stationary floating body. The rigid-body and moonpool wave amplitude

response in head and following seas are also studied numerically. We see from the results that the moonpool strongly affects heave motions in a frequency range around the piston-mode resonance frequency of the moonpool. No resonant water motions occur in the moonpool at the piston-mode resonance frequency. Instead large moonpool motions occur at a heave natural frequency associated with small damping near the piston-mode resonance frequency